This invention relates to switched capacitor converters, and more particularly to efficient gate drivers for such converters.
A switch-mode power converter is a specific type of power converter that produces an output voltage by switching energy storage elements (i.e. inductors and capacitors) into different electrical configurations using a switch network. A switched capacitor power converter is a type of switch-mode power converter that primarily utilizes capacitors to transfer energy. In such converters, the number of capacitors and switches increases as the conversion gain increases.
As used herein, conversion gain represents a voltage gain if the switched capacitor power converter produces an output voltage that is larger than the input voltage or a current gain if the switched capacitor power converter produces an output voltage that is smaller than the input voltage.
FIGS. 1-2 show two examples of switched capacitor power converters that receive an input voltage VI from a voltage source 16 and provide an output voltage VO to a load 18. Both of the examples are also known as cascade multipliers. Note that in FIG. 2, a number of the switching devices of the circuit in FIG. 1 are replaced with series of multiple devices, thereby reducing the maximum voltage across individual devices in the circuit.
In normal operating, packets of charge are pumped along a chain of diode-connected NMOS transistors M0-M5 as pump capacitors C1-C3 are successively being charged and discharged. As shown in FIGS. 1-2, phase voltages VP1, VP2 are one hundred and eighty degrees out of phase. Each of the NMOS transistors M0-M5 is diode-connected, thereby only permitting boost operation (i.e. VO greater than VI). Additionally, the efficiency is severely impacted because a significant amount of voltage is dropped across each of the transistors M0-M5 during normal operation. Therefore, there is a desire to operate the NMOS transistors M0-M5 in their ohmic region, but due difficulty and/or complexity of driving the transistors M0-M5, a combination of both PMOS transistors and high-voltage transistors are typically used.
If the transistors in the switched capacitor power converter are integrated on a single substrate then it can be desirable to use as few different types of devices as possible. For a given semiconductor process, cost is related to the number of mask layers. As the number of different types of devices in a semiconductor process increases so does the number of mask layers and hence the cost.
Furthermore, it is well-known that electrons have a higher mobility than holes in silicon. Consequently, a NMOS device with a given on-resistance has a smaller gate capacitance than a PMOS device with the same on-resistance. It is also true that a NMOS device with a given gate capacitance has a smaller on-resistance than a PMOS device with the same gate capacitance. In a power converter, it is therefore desirable to replace as many PMOS devices in the main power path with NMOS devices and replace as many high-voltage devices with low-voltage devices.